Conventionally, bumpers are provided at the front or at the rear of a vehicle such as a car for mitigating impact energy transmitted to the vehicle by absorbing the impact at the time of a collision with another vehicle. The bumper typically includes an outer cover made of a resin, a reinforcement that extends in the vehicle width direction, and a foamed resin that fills a gap between the outer cover and the reinforcement. The reinforcement is mounted on a frame of a vehicle body via an impact absorbing member that absorbs impact energy by undergoing buckling deformation.
For example, as described in Patent Document 1 or the like, hollow-extruded-shaped materials made of aluminum alloy and having a predetermined cross-sectional shape, which takes into consideration the impact absorption, have been widely used as the above-mentioned impact absorbing member.
In addition, in Patent Document 2 an impact absorbing member is proposed that is obtained by press deep drawing an aluminum plate material to form a bottomed cylindrical body and then forming a spiral-shaped step on the outer wall surfaces by press working the bottomed cylindrical body in the axial direction.